Conversion treatments lead to a superficial structural modification of the metal substrate (e.g. alloys of aluminium, titanium and other metals) by an anodisation process (an operation of electrolysis, for example chromic, sulphuric or phosphoric anodic oxidation) or by a simple chemical conversion process (for example, chromatizing or phosphatizing).
Said treatments allow a highly adherent layer of oxide (or hydroxide) to be grown, at the expense of the base metal, said layer being placed in an anode situation. On aluminium alloys, in particular, the baths of chromic acid lead to the formation of a fine (several microns) layer which is porous and exhibits a good capacity for the adhesive bonding of organic coatings.
Among the chemical conversion processes, chromatizing allows the formation of a highly adherent, thin deposit of metal chromates, by contacting the surface of the component to be treated (typically alloys of aluminium, zinc or steels) with an acidic solution based on dichromates and fluorine-containing activators. This treatment enhances the corrosion resistance of the substrate and is also used as a tie base for paints.
Because they use strong acids or bases and toxic materials such as chromates in immersion tanks, these surface treatment processes exhibit many disadvantages, particularly with regard to their harmful influence on the environment.
Other drawbacks of said surface treatment processes is the high amount of energy needed for their heating and maintenance and the fact that their use is limited to elementary parts.
Moreover, these processes require substantial amounts of water for rinsing the excess treatment solutions away from the treated components; the rinsing water and the spent process solutions must be treated in order to remove the dissolved metals, before they are disposed of or re-used; the removal of the metals produces additional toxic waste, which is difficult to purify and to dispose of.
The entirety of these treatments, subsequent to the implementation of the processes, increases the cost of use of the conventional wet-chemical processes.
Similarly, components treated at the end of their life, or in renovation phases, give rise to toxic waste which is prejudicial for the users.
Recently much stricter legislations have mandated in Europe and in the US for the progressive reduction and finally removal of the environmentally hazardous compounds, especially chromate species, making therefore urgent the need for the development of non-chromate coatings.
Consequently processes have been proposed which employ the sol-gel coating technique in order to overcome the disadvantages of the aforementioned wet-chemical processes and especially of the processes involving chromates.
Among the various techniques developed, sol-gel process is considered to be one of the most promising alternative methods to conventional chromate treatment. There are a lot of advantages inherent to the sol-gel process. First, sol-gel technology provides a low temperature chromate-free route for the preparation of coatings that are applicable to most of metallic substrates; further, the properties of sol-gel coatings can be controlled by various synthesis parameters; at last, it is possible to introduce a wide range of functional additives into the formulation, thus enabling to adjust the physical and chemical properties and to impart specific functionalities to the coatings.
Historically, the first type of sol-gel corrosion protection coatings is inorganic oxide sol-gel derived films. Various sol-gel oxide films such as SiO2, ZrO2, CeO2, SiO2/Al2O3 and SiO2/TiO2 etc. have been extensively studied to impart corrosion protection to various metallic substrates.
However, there are some limitations to said inorganic oxide sol-gel derived films due to the inorganic character of the material.
For instance, limited coating thickness owing to the crackability undermined the protection performance which restricted the applications in the aerospace industry.
To overcome those limitations, an attractive solution is to introduce an additional organic component into the inorganic sol-gel network to form a hybrid organic inorganic coating via a conventional sol-gel polymerization process using organometallic precursor compound.
Such hybrid sol-gel coatings combine the advantages of both organic and inorganic coatings.
An example of a formulation that can be used to prepare hybrid sol-gel coatings is the product known as “Boegel” developed by Boeing.
“Boegel” is a water basis diluted sol comprising GlycidyloxyPropylTriMethoxySilane (GPTMS) and Zirconium Tetrapropoxide (TPOZ) as main components, which can form a thin hybrid coating deposited on an aluminium alloy surface.
The hybrid sol-gel coatings prepared from said diluted sol intrinsically has limited anticorrosion properties.
The corrosion resistance is not provided by the sol gel coating itself but by the combination of the sol-gel coating-acting as adhesion promoter—with the paint systems.
Moreover, the methods for producing hybrid sol-gel coatings from said diluted sol involve several steps including the sol preparation and hydrolysis reaction.
Finally said sol has a limited pot life.
Some improvements to said hybrid organic inorganic sol-gel coatings are described in WO-A2-2007/003828 which discloses a concentrated sol, free of any noxious solvent and allowing the preparation of sol-gel coatings having an increased dry thickness, and a better corrosion resistance.
However, to obtain such a corrosion resistance, assessed by the neutral salt spray test, drying at a temperature above 60° C., preferably above 80° C., more preferably above 100° C. is absolutely required.
Moreover, the corrosion resistance, as assessed by the Salt Spray Test of the sol gel coatings produced in WO-A2-2007/003828 is only of about 168 hours.
On the other hand, recently, UV curing technology has been combined with hybrid sol-gel material with many advantages such as low energy consumption, high reactivity, solvent-free technology, and stability of the formulations when not exposed to UV light.
The UV technology, combined with the introduction of an inorganic phase at the nanoscale, has given birth to a variety of novel UV cured hybrid materials but the photopolymerization was generally limited to the organic part.
Interestingly, UV irradiation was also proved to be suitable to induce a sol-gel reaction through the catalysis of photoacids produced by the photolysis of onium salts
Thus, U.S. Pat. No. 4,101,513 discloses onium salts that are radiation activable catalysts for the hydrolysis of alkoxysilanes. Anhydrous compositions comprising said silanes and said catalysts are storage stable. This opens up perspective for the replacement of conventional thermal curing sol-gel process by a photoinduced sol-gel process catalysed by photoacid.
The super acids produced by photolysis of onium salts are also well-known photoinitiators of cationic photopolymerization. US-A1-2009/0318578 discloses an ultraviolet-curable coating composition comprising (A) at least one silane having a hydrolysable group and at least one group containing a cyclic ether; (B) at least one material containing one or more cyclic ether groups; which is not an alkoxysilane and is different from the silane (A); and (C) a cationic photoinitiator. In other words, the compositions of said document combine the cationic cure capability of cyclic ethers and other cationic curing materials with the cationic induced hydrolysis and subsequent condensation typical of alkoxysilanes.
Although the coatings prepared using said compositions exhibit some corrosion resistance, said resistance is actually very limited.
In addition, said patent application is silent on the mechanical properties and solvent resistance of the coatings prepared using said compositions.
US-A1-2011/0060068 discloses radiation-curable, free-radically crosslinkable formulations comprising at least one alkoxysilane and at least one acid-generating photoinitiator.
In the same way as the compositions of US-A1-2009/0318578 mentioned above, although the coatings prepared using the compositions of US-A1-2011/0060068 exhibit some corrosion resistance, said resistance is actually very limited.
In addition, said patent application is again silent on the mechanical properties and solvent resistance of the coatings prepared using said compositions.
Overall, in the methods, such as the method disclosed in US-A1-2009/0318578, involving photo sol-gel polymerization, the photolysis of a cationic photoinitiator such as a diaryl iodonium salt generates a photoacid (superacid) which then catalyzes both the cationic polymerization of a cationically radiation polymerizable resin and the sol gel polymerization of silanes precursors in the presence of water (moisture) present in the ambient atmosphere.
Hybrid sol gel films are therefore obtained.
Said methods have some advantages such as:                Single step processes (liquid precursor based film to cross-linked film);        Rapid reaction;        No water addition because hydrolysis of the silane precursors relies simply on moisture diffusion from ambient air;        1-K stable formulations until exposed to UV light;        Easy to perform.        
However, although the coatings prepared using the above formulations provide some corrosion protection on steel. There still exists a need for a solvent free, 1-K, coating having improved, very good anti-corrosion properties and also having good mechanical and solvent resistance properties.
In the light of the above, therefore, there exists a need for a radiation curable composition for preparing a hybrid sol-gel layer on a surface of a substrate, for example of a metal surface, that makes it possible to prepare a hybrid sol-gel layer that has an enhanced and high corrosion resistance as defined in particular by the salt-spray treatment test and that has also good mechanical properties and good solvent resistance.
In other words, and contrary to the known radiation curable composition for preparing a hybrid sol-gel layer, an huge enhancement to the corrosion protection of metals including neutral salt spray and filiform corrosion must be achieved without detriment to the other properties of the hybrid sol-gel coating, including, the mechanical resistance such as the scratch resistance, and wear resistance, the chemical resistance such as the solvent and hydraulic fluids.
There also exists a need for a radiation curable composition which has a low or zero solvent content, particularly in terms of noxious or toxic solvents, and in terms of other compounds that might have an adverse influence on the environment.
There exists, finally, a need for a process for preparing a 1K, solvent free hybrid sol-gel coating on a surface, for example a metal surface, that is simple, reliable, easy to carry out, which comprises a limited number of steps and treatments or coats to apply, and which can easily be integrated into the existing processes, so as to reduce workers exposures and application cycles for surface treatment of metal or composite surfaces.
The goal of the invention is to provide a radiation curable composition for preparing a radiation curable hybrid sol-gel layer on a surface of a substrate, for example of a surface comprising a metal, and a method for preparing a hybrid sol-gel layer on a surface, for example a surface comprising a metal or a metal alloy, that uses said composition, which meet the needs set out above, among others, and which satisfy the criteria and requirements mentioned earlier on above.
A further goal of the invention is to provide to provide a radiation curable composition for preparing a hybrid sol-gel layer on a surface of a substrate, for example of a surface comprising a metal, and a method for preparing a hybrid sol-gel layer on a surface, for example a surface comprising a metal or a metal alloy, that do not exhibit the disadvantages, defects, limitations and drawbacks of the prior-art compositions and methods, and which solve the problems of the compositions and methods of the prior art.